1. Field of the Invention
The present invention relates to a catalyst useful for catalytically reducing nitrogen oxides present in the combustion exhaust gas discharged from a calcination furnace, a coking furnace or a boiler or an exhaust gas from a nitric acid production plant in the present of a reducing gas such as ammonia to convert the nitrogen oxides to harmless substances, and to a method for converting these nitrogen oxides by reduction to nonharmful products over the catalyst.
2. Description of the Prior Art
The recent rapid progress of heavy chemical industries has been accompanied by undesirable problems such as air pollution caused by the accelerated evolution of nitrogen oxides. Thus, an urgent solution to the problem of nitrogen oxides evolution is desired, especially in large cities and in the vicinities of industrial areas. Under these circumstances, various methods for removing nitrogen oxides or reducing the contents of nitrogen oxides in waste gases have been proposed. Among these proposals is the so-called dry-type selective catalytic reduction method which comprises selectively reducing nitrogen oxides to harmless nitrogen in the presence of a reducing gas such as ammonia. This method is advantageous from several viewpoints. For example, a high conversion of nitrogen oxides can be obtained and the cost is much lower than the wet-type method. This method also has the advantage over the wet-type method in that such post treatments as the treatment of the waste water by-product obtained from the treatment of the waste gas, and the like need not be conducted. Moreover, process maintenance can be very easily achieved. By virtue of these advantages, this method is a practical industrial method. However, the method has the disadvantage in that the reaction temperature of the exhaust gas treatment must be as high as about 400.degree. C, and when this method is used in the treatment of the low-temperature exhaust gas discharged from a coking furnace, a calcination furnace or the like, it is necessary to heat the exhaust gas. If this is not done, then excessive portions of the reducing gas such as ammonia added to the waste gas are discharged undecomposed into the atmosphere. Therefore, this relatively desirable method possesses certain problems which must be solved.
Catalysts comprising an oxide of V, Cu or the like supported on a porous carrier have been previously used for the conversion of nitrogen oxides, to other gaseous products, but catalysts of these types have various deficiencies. For example, the activity of these catalyst is low at low temperatures and excessive portions of the reducing gas added to the exhaust gas to reduce the nitrogen oxides are discharged into the atmosphere without being reacted with the nitrogen oxides. Moreover, it is difficult to reduce the nitrogen oxides with high efficiency over these catalysts.